HFC's, being non-ozone depleting compounds, have been identified as alternative blowing agents to chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) in the production of thermoplastic foams. However, it has been found that it can be more difficult to process thermoplastic foams being blown with HFC's than with CFC's or HCFC's. For instance in the production of extruded polystyrene (XPS) foam, HFC-134a (tetrafluoroethane) and HFC-125 (pentafluoroethane) have limited solubility and higher degassing pressure in the thermoplastic resin than either CFC-12 (dichlorodifluoromethane) or HCFC-142b (1-chloro-1,1-difluoroethane). This requires foam extrusion systems to be operated at a higher pressure to keep the blowing agent in solution and prevent premature degassing before the die. The higher degassing pressure makes the foaming more difficult to control and the higher operating pressure may be too high for some extrusion systems.
Though HFCs do not deplete the stratospheric ozone many HFCs have relatively high global warming potentials (GWP) compared to atmospheric gas blowing agents like carbon dioxide (CO2) and nitrogen (N2).
The use of hydrofluoroolefins in general in blowing agent compositions has been disclosed in references including US Pat. Publication 2004089839, US Pat. Publication 2004119047, WO 2004/037913, and US Pat. Publication 2007100010. However, the specific advantages and benefits of using TFP as a blowing agent in thermoplastic resins have not been disclosed. It was discovered that while some of the compounds listed in the generic formulas given for hydrofluoroolefins in these references may be effective in blowing agent compositions for the production of low density thermoplastic foams, others are not. To those skilled in the art, it is understood that almost any volatile material with an appreciable solubility in a thermoplastic resin can provide some blowing agent activity and reduce the density of a thermoplastic resin product. However, those skilled in the art recognize that the difficulty is in finding blowing agents that are capable of producing foamed products with useful characteristics. The present invention is directed towards the discovery that TFP is a particularly effective blowing agent for thermoplastic foams including extruded polystyrene foams. TFP blowing agents can produce low density, closed-cell thermoplastic foams with controllable cell size that are useful as thermal insulating foams.
WO 2004/037913 and US Pat. Publication 2004119047 disclose a generic formula for HFOs that includes TFP but teach that fluoropropenes containing additional fluorine are preferred, specifically tetrafluoropropenes and pentafluoropropenes, and even more preferred are HFO-1234ze, HFO-1234yf, and HFO-1225ye, and any stereo isomers thereof. There is no disclosure of any advantages of using TFP. The present inventors have discovered that HFO-1234ze, HFO-1234yf, and HFO-1225ye have lower solubility in polystyrene resin than TFP and therefore are not expected to be as efficient blowing agents for polystyrene. It is known that solubility of a blowing agent in the thermoplastic resin is critical in determining its potential for use in producing low density foams. The solubility determines the quantity of blowing agent that can be incorporated into the resin, which in turn is crucial in determining the degree of density reduction achievable. These references also disclose that TFP may not be suitable for many applications due to its flammability. However, the inventors of the present invention have found TFP to be suitable for thermoplastic foaming applications, particularly of polystyrene.
US Pat. Publication 2007100010 discloses blowing agent compositions comprising components from a generic formula for unsaturated hydrofluorocarbons or hydrofluoroolefins along with an additional list of unsaturated hydrofluorocarbons. However, the examples demonstrating the use of these formulations as blowing agents in thermoplastic resin and polystyrene foams only give typical operating conditions for a foam extrusion process familiar to one skilled in the art. There are no actual examples of extrusion foaming using hydrofluoroolefins. Furthermore, there is no mention of any specific blowing agents to use. The actual examples of foaming that are given are limited to thermosetting foams such as polyurethane or polyisocyanate and demonstrate the use of longer chain HFOs where the unsaturated bond is not on a terminal carbon, including 1,1,1,4,4,5,5,5-octafluoro-2-pentene (HFO-1438mzz) and 1,1,1,4,4,4-hexafluoro-2-butene (Z-HFO-1336mzz). Those skilled in the art recognize that an effective blowing agent for polyurethane foaming systems will not necessarily be effective in thermoplastic foaming systems.
US Pat. Publication 2006243944 discloses numerous compositions comprising combinations of materials which include a combination of TFP and at least one other compound selected from a specific list including numerous HFCs and hydrocarbons. The combinations are described as being useful for a variety of uses including use as blowing agents. There is no specific disclosure of the use of TFP for thermoplastic foaming, nor are examples of TFP in blowing agent formulations disclosed. Furthermore, no formulations for producing expandable resin compositions are disclosed, and no blowing agent compositions useful for foaming are disclosed. Many compositions covered by the broad disclosure are not suitable for producing low density closed-cell foams.
U.S. Pat. No. 6,858,571 and WO 2004/037742 disclose azeotrope-like compositions that contain pentafluoropropene (HFO-1225) and a second fluid from the group consisting of 3,3,3-trifluoropropene (“HFO-1243zf”), 1,1-difluoroethane (“HFC-152a”), trans-1,3,3,3-tetrafluoropropene (“HFO-1234ze”), and combinations of two or more thereof. The use of these azeotropes as blowing agents is mentioned. However, these references do not teach preferred compositions for the foaming of thermoplastics, and many of the compositions disclosed will not be as useful as blowing agents particularly for polystyrene due to the high content of HFO-1225, which the present inventors found not to be a particularly good blowing agent for polystyrene. These references do not disclose the particular value of TFP as a blowing agent nor the benefits of TFP over HFO-1225 in terms of solubility and therefore ability to achieve a lower foam density. Furthermore they do not disclose that TFP would be useful as a blowing agent in the absence of HFO-1225, as this reference is specific to compositions including HFO-1225; in the present invention it was discovered that TFP is a particularly useful blowing agent for thermoplastic foams either by itself or in combination with other coblowing agents and cosolvents.
U.S. Pat. No. 5,205,956 discloses the use of vinyl fluoroalkanes of the formula CH2═CH—Cn-F(2n+1), where n is an integer from 1 to 6, in blowing agent formulations to replace CFC-11 and/or CFC-12. For replacement of CFC-11 and CFC-113, n is preferably from 3 to 6, and for replacement of CFC-12 n is preferably from 1 to 3. The examples disclose longer, higher boiling vinyl fluoroalkanes, including vinyl perfluoro-n-butane and vinyl perfluoro-n-hexane, in blowing agent formulations for polyurethane foaming. The present inventors found that the disclosed, preferred vinyl fluoroalkanes to replace CFC-12 in polystyrene, namely when n=2 and 3, have very low solubility in polystyrene resin and are therefore not predicted to be effective blowing agents for polystyrene. This is unexpected since TFP has a much lower boiling point than the other disclosed vinyl fluoroalkanes and therefore predicted to have a lower solubility in the resin. Furthermore, as explained by Sanchez and Rogers (1990) “Solubility of gases in polymers” Pure Appl Chem 62(11):2107-2114, it is expected that in an homologous series of gases, such as alkanes, that solubility will increase with increasing size or length. As mentioned, the inventors discovered that this wasn't the case with TFP among the vinyl fluoroalkanes tested.
The examples in the patent are limited to polyurethane foam manufacture. No examples of manufacture of a thermoplastic foaming are provided.
U.S. Pat. No. 4,085,073 discloses blowing agent compositions containing chlorofluoromethane, bromochlorodifluoromethane, and mixtures thereof. Alone or with minor amounts of other halogen containing carbon compounds as blowing agents. The patent mentions TFP, along with a list of other halogenated compounds as possible diluents. The use of TFP as a blowing agent itself or as a significant fraction of the blowing agent formulation is not disclosed.
GB 950,876 discloses a process for the production of polyurethane foams. It describes that any suitable halogenated saturated or unsaturated hydrocarbon having a boiling point below 150° C., preferably below 50° C., can be used as the blowing agent. TFP is among the halogenated hydrocarbons listed however the examples all employ CFC-11 as the blowing agent. There is no disclosure related to blowing agents for thermoplastic foaming.
Historically, chlorofluorocarbon (CFC) compounds, such as CFC-12 (difluorodichloromethane) and CFC-11 (trichlorofluoromethane), and hydrochlorofluorocarbon (HCFC) compounds, such as HCFC-22, (chlorodifluoromethane), HCFC-141b (1,1-chloro-1-fluoroethane), and HCFC-142b (1-chloro-1,1-difluoroethane), were preferred blowing agents for the production of thermoplastic foams such as extruded polystyrene (XPS) foam. However, due to concerns with ozone depleting compounds and with the adoption of the Montreal Protocol for the protection of the stratospheric ozone layer, there has been a need to identify non-ozone depleting compounds to replace the ozone depleting CFC and HCFC blowing agents. The fluorocarbon industry was successful in developing new alternatives such as HFC-134a (1,1,1,2-tetrafluoroethane), HFC-32 (difluoromethane), or HFC-152a (1,1-difluoroethane) in blowing agent compositions for thermoplastic foaming.
Though HFCs do not deplete stratospheric ozone they generally still have a high global warming potential (GWP). Concern over climate change and global warming has increased the importance of developing low-GWP, non ozone depleting compounds that are useful in blowing agent compositions for thermoplastic foaming.
Furthermore, in the production of insulating foam it is desired to have low density foamed product that maintains a high, long term R-value (insulating value). Though HFC-134a can provide long term R-value it is not soluble enough in polystyrene to produce foam product to the same low density as with using HCFC-142b or CFC-12. HFC-134a also has a very high nucleation density such that foams produced with it tend to have a very fine cell structure, which is not desired in all applications. HFC-152a and HFC-32 can be used to produce lower density foams with larger cell sizes than with HFC-134a but their high diffusivities in polystyrene will result in foams that age faster and don't possess the same long term R-value.